The invention proceeds from a process for purifying alkanesulfonic acids.
Alkanesulfonic acids, in particular methanesulfonic acid (MSA), are employed in myriad applications in pure form and in admixture with water and other solvents. The use of MSA is particularly widespread in electroplating, tinplate production and wire tinning. Alkanesulfonic acids are also employed as solvent or as catalyst in alkylation and esterification reactions for example. A further field of application for alkanesulfonic acids is the production of biodiesel where the typically employed sulfuric acid may be replaced by alkanesulfonic acids on account of the improved performance properties of the latter.
Alkanesulfonic acids are also an alternative to phosphoric-acid-containing cleaning product formulations. Since methanesulfonic acid in particular forms readily soluble salts and is readily biodegradable, the alternative use of alkanesulfonic acid can play a role in water pollution control.
Hereinbelow, all components of crude alkanesulfonic acid other than alkanesulfonic acid and water are encompassed by the term “impurities”. The term “low boilers” is to be understood as meaning water and all components having a boiling point below the boiling point of alkanesulfonic acid. The term “high boilers” is to be understood as meaning all components having a boiling point above the boiling point of alkanesulfonic acid. The alkanesulfonic acid is in particular methanesulfonic acid.
The production of alkanesulfonic acids initially generates crude alkanesulfonic acid. This is a mixture of alkanesulfonic acid, low boilers and high boilers, the low boilers and high boilers varying depending on the production process. Low boilers are generally water, nitric acid, hydrochloric acid, thioesters, alkanesulfonyl chloride, sulfur trioxide, alkanes and alkylsulfones. High boilers often include sulfuric acid, alkanedisulfonic acids or chloroalkanesulfonic acid. Color-conferring substances may also be present.
To obtain pure alkanesulfonic acid or aqueous solutions of alkanesulfonic acid the crude alkanesulfonic acid is typically purified by distillation, nanofiltration, selective absorption of impurities over exchange resins or selective precipitation of impurities as salts. Of these, distillation is the dominant process, stripping being regarded as a distillative or evaporative process and distillation typically being performed at pressures below atmospheric pressure, since alkanesulfonic acid may form decomposition products at the temperatures required for distillation at atmospheric pressure. For example, distillative purification of methanesulfonic acid may lead to the formation of methyl methanesulfonate. A further problem is that methanesulfonic acid is corrosive at the high temperatures necessary and only a limited choice of stable materials of construction is available.
WO-A 00/31027 discloses the production of alkanesulfonic acid by oxidation of alkyl mercaptans, dialkyl disulfides or dialkyl polysulfides with nitric acid. This generates nitrogen oxides, water and further byproducts such as sulfuric acid. The nitric acid is regenerated from the nitrogen oxides by oxidation with oxygen and recycled into the process. To purify the product, low boilers and high boilers are removed by distillation in two stages to obtain pure, practically anhydrous alkanesulfonic acid. Water and nitric acid are removed from the crude product in a water removal column operated as a stripping column at slightly reduced pressure. The bottoms product obtained comprises 1 wt % water and about 1 wt % high boilers, especially sulfuric acid. The removal of the high boilers is achieved by distillation of the alkanesulfonic acids with purities of greater than 99.5 wt % and sulfuric acid contents of less than 50 ppm under high vacuum, i.e. at a pressure of from 0.1 to 20 mbar (abs).
WO-A 2015/086645 describes the production of alkanesulfonic acid by oxidation of dialkyl disulfides with nitrogen oxides. The nitrogen oxides are regenerated with oxygen-enriched air for example. The reaction products are subsequently freed of low and high boilers via two distillation columns. The thus purified product comprises an unspecified concentration of methanesulfonic acid.
GB-A 1350328 describes the synthesis of alkanesulfonic acids by chlorination of alkyl mercaptans or dialkyl disulfides in aqueous HCl. The product of the reaction is alkanesulfonic acid in 70 to 85 wt % purity. This document describes a two-stage process for producing anhydrous methanesulfonic acid. This comprises a first step in which water is distilled off and a second step in which the methanesulfonic acid is distilled out of the bottoms product with a short column and obtained overhead.
WO-A 2005/069751 describes the synthesis of methanesulfonic acid from sulfur trioxide and methane via a free-radical chain reaction with for example Marshall's acid as free-radical initiator. In this synthesis, anhydrous methanesulfonic acid is formed, but no information is given about purification. WO-A 2015/071365 describes a similar process, with distillation being suggested for purifying the resulting methanesulfonic acid. The product of this production process is mostly free from water. However, it comprises sulfur trioxide.
CN-A 1810780 describes the synthesis of methanesulfonic acid by reaction of ammonium sulfite with dimethyl sulfate. This affords ammonium methylsulfonate and ammonium sulfate. Addition of calcium hydroxide forms soluble calcium methylsulfonate and insoluble calcium sulfate which may be removed easily. Sulfuric acid is added to liberate methanesulfonic acid and once again form and precipitate calcium sulfate. The aqueous solution formed is initially subjected to distillation to remove water and then subjected to distillation under reduced pressure to obtain methanesulfonic acid.
DE-C 197 43 901 describes the synthesis of methanesulfonic acid by reaction of sulfite ions with dimethyl sulfate. These sulfite ions are reacted in an aqueous system at elevated temperature and exposed to a strong acid. Sulfate is formed as a byproduct, for example in the form of sodium sulfate. Purification of the acid is by distillation.
EP-A 0 675 107 describes a process for continuous production of alkanesulfonyl chloride (ASC) or alkanesulfonic acid (ASA) by reacting an alkane mercaptan or a dialkane disulfide with chlorine in aqueous hydrochloric acid at elevated pressure. Hydrogen chloride (HCl) and other low boilers not condensable under the process conditions are desorbed after decompression of the superatmospheric pressure. ASC is produced at a preferred temperature range of from 10° C. to 35° C. and purified by means of a distillation column. ASA is obtained from ASC by hydrolysis at temperatures of from greater than 80° C. to 135° C. in the presence of water. The purification of ASC and/or ASA is also carried out with a vapor stripper for example, residual ASC also being hydrolyzed therein.
The removal of water from aqueous methanesulfonic acid by evaporation of the water in a falling film evaporator at reduced pressure is described in U.S. Pat. No. 4,450,047. Water is drawn off overhead and a product stream comprising more than 99.5 wt % methanesulfonic acid is obtained.
U.S. Pat. No. 4,938,846 discloses the removal of water from aqueous methanesulfonic acid by evaporation of the water in two falling-film evaporators arranged in series and both operated at reduced pressure.
The disadvantage of the prior art distillation processes is that the process is highly energy intensive on account of the high temperatures and the required reduced pressure. In addition, it is not possible to remove high boilers such as sulfuric acid without particularly energy intensive conversion of the alkanesulfonic acid into the gas phase. Also, certain purification processes achieve the distillation task with falling film evaporators which are useable on a large industrial scale only with difficulty.
U.S. Pat. No. 4,035,242 discloses a likewise very energy intensive process where aqueous methanesulfonic acid is purified in a two-stage distillation process. In the first distillation column a large part of the water is removed as a low boiler at reduced pressure. The bottoms product comprising methanesulfonic acid is evaporated and separated in a second rectification column at reduced pressure to obtain the methanesulfonic acid.
U.S. Pat. No. 6,337,421 discloses the removal of sulfuric acid from methanesulfonic acid using basic anion exchange resins. Other processes of removing sulfuric acid are also described, for example distillation or fractionating crystallization and also separation by nanofiltration, but none of these achieve adequate results according to the description of U.S. Pat. No. 6,337,421.
The purification of methanesulfonic acid comprising oxidizable compounds is described in EP-A 0 505 692 and EP-A 0 373 305. EP-A 0 505 692 discloses supplying chlorine to convert the impurities into methanesulfonyl chloride which is hydrolyzed to afford methanesulfonic acid and HCl in a further step. EP-A 0 373 305 discloses supplying ozone which converts methyl thiosulfate into methanesulfonic acid. However, the disadvantage of these two processes is that high-boiling components such as sulfuric acid cannot be removed, thus necessitating further purification steps.
The fractionating crystallization of methanesulfonic acid and also of ethanesulfonic acid is known in principle from R. A. Craig et al., J. Am. Chem. Soc., 1950, Vol. 72, pages 163 bis 164 or A. Berthoud, Helv. Chim. Acta, 1929, Vol. 12, page 859, but no indication is given as to how the processes described therein could be implemented in production and purification processes on a large industrial scale.